This invention relates to communication systems, including but not limited to communication mode changes for communication units served by radio frequency communication systems.
Radio frequency (RF) communications systems are known. Many RF communication systems offer multiple modes of communication within a single system. These modes include trunked communications, conventional communications, cellular communications, and talk-around communications, all of which are manually selectable by the user of an RF communication unit, such as a mobile or portable radio. In large, managed RF communication systems or networks, such as public safety or utility systems, communication units, under normal conditions, are in communication on a known path with a central dispatcher who is responsible for the tracking of units and the coordination of their workloads.
A problem arises when a communication unit user manually selects to move his communication unit to a different communication mode, such as talk-around, conventional, or roaming into another independent communication system, which mode currently does not facilitate tracking or any other method of locating of the communication unit. The problem exists because no dispatcher of the home communication system, i.e., the communication system where the user normally is registered, knows where the communication unit is within the communication system or within another communication system. The infrastructure of the communication system does not know that the communication unit has its current communication resources, where the communication unit has moved, or when the communication unit left the communication mode last associated with the home communication system.
This situation becomes a problem when the infrastructure of the communication system needs to find the communication unit. In a cellular communication system, the infrastructure of the communication system knows that the subscriber has left the system when the infrastructure tries to call the subscriber and the subscriber does not answer. Currently, in public safety communication systems, a user must typically ask permission from the dispatcher to leave and the dispatcher must manually keep track of this information. This method can be both clumsy and cumbersome, especially when there is a lot of traffic on a communication system during any period of time. In a closed trunked system, a dispatch interrupter priority scan feature may help to find the communication unit when its user has manually selected to change to a different communication resource, providing that the new resource is still part of the original system. If, however, the communication unit goes outside the system, the unit will not be able to be found using these particular methods. Hand-off methods exist such that when a communication is handed off from one communication site to another within the same communication system, the user is de-affiliated with the old site around the same time as becoming affiliated with the new site. Also known in the art is that a communication unit automatically notifies or de-registers with its communication system when the radio is turned off. In addition, it is known for a communication unit to register in a new system when the unit roams into the new system.
Accordingly, there is a need for a method of keeping track of communication units when they change communication modes.